Temporal and spatial redundancy can be exploited using prediction to make a compact representation of the video signal possible. Pixel prediction is an important part of video coding standards such as H.261, H.263, MPEG-4 and H.264 [1]. In H.264 there are three pixel prediction methods utilized namely intra, inter and bi-prediction. Intra prediction provides a spatial prediction of the current block from previously decoded pixels of current frame. Inter prediction gives a temporal prediction of the current block using a corresponding but displaced block in a previously decoded frame, e.g. reference block. Bi-directional prediction gives a weighted average of two inter predictions.
The inter prediction method of H.264 can achieve fractional-pel resolution in the motion estimation. A fixed half-pel filter with filter taps [1 −5 20 20 −5 1]/32 is first applicable to obtain initial half-pel resolution. A bilinear filter can then be applied on a full-pel sample and a half-pel sample to achieve quarter-pel resolution. These fractional-pel interpolation filters used in H.264 are fixed, implying the same filter taps are used regardless of which particular block that is encoded.
Many techniques have been suggested to improve the inter prediction over the years, for instance by employing adaptive interpolation filters. Such an approach has been suggested by first determining displacement vectors using the fixed H.264 interpolation filter for each image block to be coded. With the obtained displacement vectors, a calculation of adaptive filter coefficients of separable 6-tap adaptive filter is then done afterwards. As a consequence, three horizontal adaptive interpolation filters and twelve vertical adaptive interpolation filters are calculated through a prediction error minimization.